Cutting tool for the continuous machining of metals and the method of making same

ABSTRACT

A cutting tool made of a material having a high red hardness for the continuous machining of metals and having a geometry determined by the change in measurements of a sample of the metal being machined when the sample is externally loaded to failure. The sample is subjected to a tensile load to produce the failure. Elongation and neck down are caused in the failure area according to the ductility of the metal. The effective cutting or rake angle of the cutting tool is established as a function of the elongation and neck down of the sample. The relief angles of the cutting tool geometry are determined to maintain a self-sharpening cutting edge under the abrasive characteristics of the workpiece material and erosion characteristics of the cutting tool material for given machining conditions. 
     The effective rake angle of the cutting tool is also established as the minimum angle producing chips of maximum hardness during the machining operation.

This is a division of application Ser. No. 304,771, filed Nov. 8, 1972,which was a continuation-in-part of application Ser. No. 45,357 filedJune 11, 1970, now abandoned.

SUMMARY OF THE INVENTION

This invention relates to a cutting tool for the continuous machining ofmetal such as the type used in a lathe, planer, shaper, or the like, andparticularly to a method of defining the correct geometry for a cuttingtool for a particular machining operation, a cutting tool having ageometry so defined, and a method of machining the metal with thecutting tool.

Heretofore the primary considerations in determining cutting toolgeometries have been to look to the cutting tool for direction as towhat these geometries should be. For example, prior art tools made ofcarbide, such as tungsten carbide, titanium carbide, tantalum carbide,or the like, have been made with negative rake angles and very smallrelief angles for machining nearly all materials. The reasons usuallygiven for this are that the carbides have extremely high red hardnessand compressive strength. The negative geometry creates cuttingpressures which are directed into the body of the carbide tool thusutilizing the extreme compressive strength of the carbides. Low reliefangles also add support to the cutting edge. This philosophyromanticizes the compressive capability of the carbides and createsconditions of inefficient cutting which generate such excessive heatsand pressures that only a tool having the high red hardness andcompressive strength of carbide could survive. In addition, thecompressive forces directed at the tool result in equal and oppositeforces being directed into the workpiece. Minute surface damage thusgenerated can be a source of catastrophic failure in parts which aresubsequently subjected to extreme conditions of stress such asvibrational stress or corrosive stress.

This invention represents a complete departure from this old philosophy.According to this invention, the proper rake angles are determined bythe material to be machined and not by the cutting tool. It is thecharacteristics of the workpiece that teaches the proper tool geometryand this is determined by the physical effect of externally loading asample of the metal workpiece to failure.

The cutting of metal by a cutting tool is a form of failure. Unless thecutting tool can create the failure, no cutting takes place. If anexcessive amount of work is required to produce the failure, heat andwear and tear on the cutting tool, overheating of the chip cut from theworkpiece, and damage to the surface of the workpiece can occur. If themetal failure created by the cutting process can be accomplished withminimum work, there will be minimum heat and wear and tear on thecutting tool, a lower chip temperature, and minimum damage createdwithin the surface of the workpiece. The result is optimum cuttingefficiency and longer tool life.

The amount of work required to cut metal with a cutting tool for a givenset of cutting conditions varies with the geometry of the cutting tool.In this invention, a sample of the metal to be cut is subjected to atensile load that will cause failure of the metal sample. Measurementsof the changes in dimensions resulting from failure of the sample aremade. From these measurements the proper tool geometry is calculated.

Tension tests are used to establish the proper cutting tool geometrybecause although the failure in metal during cutting is in compressionin the immediate area of the cutting edge of the cutting tool, the cutmetal is immediately directed away from the cutting area and istherefore unrestrained, avoiding metal interference that exists inconventional compressive loading. Accordingly, the machining of aworkpiece produces the same type of granular slippage as is produced bytensile loading of a sample of the workpiece, and in this importantrespect the types of failure are the same.

Hence, according to this invention a test bar of the same material asthe workpiece to be machined is loaded under tension to failure. Whenthe tensile forces become of sufficient magnitude to cause the metal tofail, the area at the failure is in the form of a cup cone with theperipheral surfaces of the cup cone being at a fixed angle to the axisof the metal measured in the direction of the applied tensile forces.This angle is a constant for a given metal and is generally accepted asbeing forty-five degrees. Also, the cup cone area exhibits high hardnessbecause maximum stress levels are attained at the cup cone break. Thesemaximum stress levels result in maximum work hardening of the sample inthe cup cone area. Prior to failure, elongation and neck down of themetal test bar occur. The precent of elongation and the angle of neckdown vary with different metals, generally depending on the ductility ofthe metal. However, for a given metal, the percent elongation and theneck down angle are substantially uniform when several samples of thesame material are tensile loaded to failure.

Since there are two physical changes in the geometry of the testspecimen, elongation and neck down, there are two forces that areconsidered relevant. One is the tensile force producing elongation, andthe other is the internal attractive force within the metal that causesneck down. These forces are at right angles to one another since themetal creates the neck down through internal attraction at a right angleto the direction of the elongation of the metal.

Before failure occurs under tensile loading of the test bar, thematerial in the neck down area must be deformed a specified minimumamount which is the result of the elongation and neck down. If it is notso deformed, failure will not occur. Similarly, before failure can occurin the metal workpiece being machined, the metal being cut must bedeformed that same minimum amount. Therefore, according to thisinvention the cutting angle of the cutting tool required to deform themetal of the workpiece the required minimum amount to produce failure,is a function of the neck down angle and the elongation of the testspecimen when loaded under tension to failure. This cutting angle iscalled the effective rake angle.

As a corollary, because the test sample in the area of the cup conewhere failure occurs has been work hardened to its maximum hardness, andbecause machining of metal produces this same type of failure, theproper effective rake angle C may also be defined as the minimum angleproducing the maximum hardness of chips during machining.

Another aspect of this invention involves the proper selection of therelief angles of the cutting tool. Just as with the rake angles, therelief angles of the prior art tools were chosen to be as small aspossible in order to maintain the greatest support for the cutting edge.This has been particularly true of carbide cutting tools. In contrast,the cutting tool of this invention is made self-sharpening byrecognizing the abrasive characteristics of some workpiece materials andproperly selecting the side and end relief angles on the cutting tool soas to maintain the cutting edge by abrasive action of the workpiecematerial during the machining process.

To strengthen the cutting edge of the tool, a thin wear land is formedjust beneath the cutting edge and another at the end of the cuttingedge, these lands having a substantially zero relief angle. As the toolcuts the workpiece, the chip being formed tends to erode the top surfaceor face of the tool at the cutting edge to form a crater, and thesurface of the workpiece tends to erode the wear lands. These erosionsbecome greater as machining continues. The rates of these erosions willnot necessarily be the same and will depend on the tool and workpiecematerials and the feed and speed used in machining. However, for givenmaterials and given feed and speed, the rate of erosion of the face of agiven tool will be constant and the rates of erosion of the wear landswill be constant although not necessarily equal. If during machining,the widths of the wear lands, due to an imbalance in erosion, becomesmaller, the cutting edge will become too sharp, be unsupported, andbreak off. On the other hand, if the widths of the wear lands increase,a point will be reached where the excessive wear lands will causerubbing, chattering, and tool failure. But if the widths of the wearlands become constant during the machining process, the erosion of thetop face balanced with the erosions on the wear lands, the tool becomeseffectively self-sharpening for greatly increased tool life. This can beachieved by proper selection of the side and end relief angles, and itis a feature of this invention that the relief angles are so selected.

Hence, it is a primary object of this invention to produce a cuttingtool having a geometry that cuts a metal workpiece with minimum work andmaximum efficiency for greatly improved tool life, and to define theprocedures for determining that geometry.

DESCRIPTION OF THE DRAWINGS

All figures are generally diagrammatic.

FIG. 1 is a fragmentary plan view of a lathe cutting tool of thisinvention and of the kind having primary and secondary rake angles;

FIG. 2 is a front view of the tool of FIG. 1;

FIG. 3 is a left side view of the tool of FIG. 1;

FIG. 4 is a fragmentary isometric view of the tool of FIG. 1, butwithout side and end cutting edge angles or relief angles, and showingthe cutting tool geometry as it relates to a coordinate system of theworkpiece;

FIG. 5 is a side view of a metal test bar sample of a material identicalto that to be cut;

FIG. 6 is an enlarged fragmentary side view of the test bar sample ofFIG. 5 following failure under tensile loading;

FIG. 7 is a side view of another test bar sample having an area ofreduced cross section to isolate the plane where failure will occur;

FIG. 8 is a side view of the test bar sample of FIG. 7 following failureunder tensile loading;

FIG. 9 is a three-dimensional geometric diagram illustrating therelationship between the primary, the secondary, and the effective rakeangles, and the relationship between the applications of forces in ametal failure;

FIGS. 10, 11, and 12 are side views of cutting tools shown cutting aworkpiece for the purpose of illustrating practical limitations in themagnitude of the secondary rake angle;

FIG. 13 is a front view of a lathe tool of another embodiment of thisinvention;

FIG. 14 is a right side fragmentary view of the tool of FIG. 13;

FIG. 15 is a fragmentary plan view of the tool of FIG. 13;

FIG. 16 is an enlarged view of the tip portion of FIG. 13;

FIG. 17 is a composite diagrammatic drawing showing a lathe tool bit, aworkpiece, and a chip with a tensile test specimen shown prior to andafter failure superimposed thereon for the purpose of explaining thetheory of the invention;

FIG. 18 is an enlarged view of the central portion of FIG. 17; and

FIG. 19 is a graph showing the relationship between the effective rakeangles of cutting tools versus the percents true elongation of materialswhen tensile loaded to failure for both tools of this invention andprior art tools.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention is primarily directed to a method for defining thecorrect geometry of cutting tools adapted for the continuous machiningof metal, to cutting tools having geometries so defined, and to a methodof machining the metals with the cutting tools. In its preferredembodiments, the invention is directed to such tools made of a materialhaving a high red hardness. By this it is meant that the temperature atwhich these tools maintain their cutting effectiveness is in excess ofapproximately 1,200° F. Such materials embrace the carbides includingthose of tungsten, tantalum, titanium, etc.; ceramics such as thosebased on alumina, silica, and the like; natural and synthetic diamond;etc. Of these materials, all generally known to the art, the carbidesrepresent a particularly preferred class. Such materials are treated innumerous references, among which may be cited "Machining Data Handbook,"2nd edition, Metcut Research Associates, Inc., Cincinnati, Ohio, 1972,pp. 789-793.

Although the principles of this invention, as the same are describedhereafter, will be understood to apply generally to tools adapted forthe continuous machining of metal, such as for example lathe tools,drills, end mills, reamers, broaches, and the like, the descriptionherein is directed to lathe tools for ease in understanding theinvention. By "continuous machining" here is meant the more or lessconstant machining of metal as occurs in machine tools, although acontinuous chip may or may not be formed. In "continuous machining ofmetal" a substantially constant velocity is maintained between theworkpiece being cut and the cutting tool throughout a cutting operation.Tools used in such machining include, for example, lathe tools,broaches, drills, end mills, and so forth. It will be appreciated thatthe cutting tools of this invention are adapted for the "continuousmachining of metal," as in a power operated machine, and hence willordinarily include a support section, such as a shank or rod, which maybe gripped in a cutting tool holder of the machine, such as in the toolholder of a lathe. Rigidly attached to the supporting portion of thetool is the cutting portion of the tool having a cutting edge andcutting angles defined as hereinafter described. In some instances thecutting portion is silver soldered or otherwise attached to thesupporting portion of the cutting tool. In other instances, such as thepreferred embodiments described herein, the cutting tool is of a singlematerial, one end thereof being adapted for mounting in a metal workingmachine and the other end including a cutting edge for cutting a metalworkpiece. It will be noted that each of these tool configurations ischaracterized by including a support portion for attachment of the toolin a metal working machine, and a cutting portion which includes acutting edge and cutting angles as defined herein.

FIGS. 1, 2, and 3 illustrate a lathe cutting tool 20, which is alsoshown in FIG. 4 cutting a workpiece W but without any clearance andrelief angles so that the locations and determinations of primary,secondary, and effective rake angles can be illustrated and described.The geometry of the tool 20, including its various angles, will bedescribed relative to the workpiece, as is customary in the art, using athree-dimensional X, Y, Z coordinate system. The coordinates XY define aplane through the point of the tool and axis a of the workpiece W; thecoordinates XZ define a plane through the point of the tool and normalto the XY plane and parallel to the axis a; and the coordinates YZdefine a plane through the point of the tool and normal to both the XYand XZ planes. Tools having geometries defined relative to othercoordinate references can be easily transformed mathematically to thecoordinate system used herein. The tools of this invention will bedescribed without chip breakers, which are known in the art, althoughthese can be provided if desired.

The tool 20 has a shank 21 and a nose 22, the nose 22 having a cuttingpoint 23, a face 24, a base 25, an end relief surface 26, a side 27, anda side relief surface 28. The nose 22 also has a cutting edge 30 and anupper front edge 31.

As viewed in FIGS. 1 through 3, and relative to the coordinate system ofFIG. 4, the horizontal angle that the cutting edge 30 makes with avertical plane parallel to the YZ plane and shown by a line 32 is theside cutting edge angle designated as the angle g, and the verticalangle that the side relief surface 28 makes with the vertical plane 32is the side relief angle designated as the angle h. The horizontal anglethat the upper front edge 31 makes with the vertical plane XZ, shown bythe line 35, is the end cutting edge angle designated as the angle i,and the vertical angle that the end relief surface 26 makes with thevertical plane 35 is the end relief angle designated as the angle j.

The angle the upper front edge 31 makes with a horizontal plane parallelto the XY plane and shown by a line 36, prior to providing the endcutting edge angle i, as shown in FIG. 2, is the primary cutting or rakeangle A. The angle the cutting edge 30 makes with the same horizontalplane shown by a line 37 prior to providing the side cutting edge angleg, as best shown in FIG. 3, is the secondary cutting or rake angle B.While the angle B is shown to be negative in accordance with standardtool nomenclature, the cutting edge 30 sloping downwardly toward thepoint 23, the angle B could also be positive, the cutting edge 30sloping upwardly toward the point 23, only the magnitude of the angle Bbeing of consequence within the limitations to be described.

The effective rake angle C is illustrated in FIG. 4. It is determined bycalculating the resultants of the primary and secondary rake angles Aand B. On the tool 20 of FIG. 4, the effective rake angle C is measuredbetween the X axis and a line 40 drawn on the same plane as the face 24of the tool 20. The magnitude of the effective rake angle C isdetermined by the formula cos C = cos A cos B as is commonly known inthe art.

It is to be understood that not all tools have primary and secondaryrake angles, some having only a primary (effective) rake angle. In thosehaving no secondary rake angle, the secondary rake angle B being zero,cosine B is equal to one, and hence the effective rake angle C is equalto, or the same as, the primary rake angle A.

In the orientation of the tool in FIGS. 1 through 4, the workpiece beingmachined moves past the tool in a downward direction as viewed in FIGS.2 through 4, and the tool is fed to the right as viewed in FIG. 2 duringmachining. The nomenclature heretofore used in describing the tools ofFIGS. 1 through 4 will also be used hereinafter where appropriate indescribing tools of other embodiments of this invention.

The manner of determining the magnitude of the effective rake angle C,which is the most critical angle, in accordance with this invention willnow be described.

The cutting or machining of material from a metal workpiece is achievedby producing a failure in the metal. Unless the cutting tool can createthe failure, no cutting takes place. It has been found that tool life isdirectly related to the amount of work required to produce the failureduring the machining process. For a given workpiece and for a givenmachining operation, the more work required to produce failure, theshorter the tool life, and conversely the less work required to producethe failure, the greater the tool life. Hence, the greatest tool life isachieved where the cutting or failure is accomplished with minimum workand maximum efficiency.

The amount of work required to cut metal with a cutting tool for a givenset of cutting conditions, such as feed and speed of cutting, varieswith the geometry of the cutting tool. It is a basic concept of thisinvention that the geometry of the cutting tool is determined directlyfrom the characteristics of the metal being cut and specifically from asample of that metal subjected to a tensile load that will cause failureof the metal sample. Certain of the changes that take place in the metalsample when loaded to failure are directly related to the changes thattake place in the metal workpiece when cut or machined. Because of theserelationships, the effective rake angle C can be determined frommeasurements of the changes in the physical characteristics resultingfrom failure of the sample under tensile load. A theory will be advancedfor why these relationships hold true, but first the procedure fordetermining this angle in accordance with this invention will bedescribed.

FIG. 5 shows a bar 45 with ends 46 and 47 that are held in a tensileloading device (not shown) to load the bar 45 until it fails. Two marks56 and 57 are made on the bar to define an initial length l_(i) of a barsection prior to loading. FIG. 6 shows diagrammatically the condition ofthe bar 45 at failure. Characteristically, the two sections 48 and 49created by the break have elongated, and on opposite sides of the breakthere are neck down sections 50 and 51, the surface angle of each neckdown section measured from the axis of the applied tensile force beingdesignated as angle D. Where the actual break occurs, an internalfrusto-conical well 52 is formed on the part 48 and a complementaryexternal frusto-conical section 53 is formed on the end of the section49. The marks 56 and 57 have elongated to positions 56' and 57' on thesurface of the neck down area. The distance between the marks indicatedat 56' and 57' in FIG. 6, measured parallel to the axis of the bar 45 isl_(f) . However, the true distance between the marks 56' and 57' locatedwithin the neck down area is l_(t).

In accordance with this invention the effective rake angle C of thecutting tool is determined by the formula cos C = (l_(i) /l_(f)) cos D.The angle D can be measured directly from the test bar of FIG. 6, andl_(i) from the test bar of FIG. 5; however, for l_(f) there is noassurance that the marks, such as the marks 56 and 57 when elongatedwill lie within the neck down section, and it is the measurement ofelongation within the neck down or failure area that is required. Inaddition, a bar of uniform cross section, such as the bar 45, may haveseveral neck down or partial neck down areas, making measurements ofelongation unreliable.

To obtain a more accurate measure of elongation, a bar such as the bar62 shown in FIGS. 7 and 8 is used. The bar 62 has an annular groove 63to localize the area where failure will occur in tensile loading. Theends 64 and 65 of the bar 62 are clamped in a tensile loading device(not shown) to produce the break 66 illustrated in FIG. 8, separatingthe bar 62 into two sections 67 and 68. The distance between edges 69and 70 of the annular groove 63 before elongation is l_(i), and thedistance between these edges 69 and 70 after elongation is l_(f) asindicated in FIG. 8. Since the bar 62 has a localized area (the annulargroove 63) where failure will occur, l_(i) and l_(f) can be accuratelymeasured.

Hence, the dimensions l_(i) and l_(f) are determined by loading the bar62 to failure and measuring the length of the notch before and afterloading, and the angle D is determined by loading the bar 45 to failureand measuring the neck down angle D directly. The effective rake anglecan then be calculated from the formula cos C = (l_(i) /l_(f)) cos D.

FIG. 9 illustrates vectorially the geometric relationship between l_(i),l_(f), and l_(t) with respect to three-dimensional coordinate axes X, Y,and Z with the X coordinate being parallel to the axis of the testspecimen. If the metal sample of length l_(i) is deflected by a forcef_(xy) through an angle E, a new length l_(f) is reached. Since thedifference between l_(f) and l_(t) is caused by neck down, and neck downis produced by an internal force in the metal at right angles to theapplied tensile force, a force f_(xz) applied to the new length l_(f)through the angle D creates a new length l_(t). The forces f_(xy) andf_(xz) are at right angles to one another, f_(xy) being in the XY planeand f_(xz) being parallel to the XZ plane and perpendicular to the XYplane. The resultant of these forces establishes the angle C betweenl_(i) and l_(t), the initial length before elongation and the truelength after failure.

Therefore, it can be seen from FIG. 9 that cos C = l_(i) /l_(t) ; cos E= l_(i) /l_(f) ; cos D = l_(f) /l_(t) ; or cos C = cos E cos D.

Referring again to the lathe tool 20 of FIGS. 1 through 4, where thetool, such as the tool 20, has both a primary rake angle A and asecondary rake angle B such that cos C = cos A cos B as heretoforedescribed, because cos C in accordance with this invention is equal tocos E cos D, the primary rake angle A can be made equal to the angle Eand the secondary rake angle B can be made equal to the angle D.However, this is not critical. It is only critical to define C by theformula cos C = cos E cos D and once so defined the angles A and B canbe varied within practical limits so long as the products of theircosines is equal to cos C.

These practical limits relate to the angle B, and can be best explainedby referring to FIGS. 10 through 12.

FIG. 10 shows a cutting tool 71 having a cutting edge 72 and a secondaryrake angle equal to zero. The tool 71 is shown cutting a metal workpieceW with the depth of cut, or chip depth, shown by d. The direction ofrotation of the workpiece W is shown by the arrow 73, and the tool isviewed looking into the headstock of the lathe with the tool feedingtoward the headstock. With a secondary rake angle B equal to zero, thecutting tool 71 introduces a force M into the workpiece W and in adirection generally limited to the chip area or chip depth so that verylittle energy is wasted in unnecessarily deforming the workpiece. Aforce M', equal and opposite to the force M, is directed as shown intothe point of the tool. An angle B of zero is considered a satisfactorycompromise which generally limits the deformation of the workpiece tothe chip area while utilizing the compressive strength of the tool toreduce the possibility of chipping or breakage at the point andproviding sufficient mass in the cutting area to conduct heat away fromthe cutting edge.

FIG. 11 is generally the same as FIG. 10 except it shows a tool 74having a cutting edge 75 and a positive secondary rake angle B. Withangle B positive, the tool 74 has the advantage over the tool 71 in thatit offers more assurance that the deformation of the workpiece W will belimited to the chip area as shown by the direction of the force N, buthas the disadvantage in introducing a force N', equal and opposite tothe force N, into the point of the tool which tends to chip or break thepoint. Also, as the angle B becomes more positive, there is less mass atthe cutting area of the tool to conduct heat away from the cutting edge.While a small positive angle B is desirable to insure a minimum ofwasted energy caused by unnecessary workpiece deformation, its magnitudeis limited due to the decrease in compressive capability at the point ofthe tool, and the decrease in mass to conduct heat, as the angle Bbecomes more positive.

FIG. 12 is much like FIGS. 10 and 11 except that it shows a tool 76having a cutting edge 77 and a negative secondary rake angle B. With anegative angle B the cutting tool 76 introduces a force R into theworkpiece as shown, which in contrast to the tools 71 and 74, deforms anarea 78 of the workpiece beyond the depth of the chip. This results inwasted energy and represents a disadvantage over the tools 71 and 74.However, the tool 76 offers an advantage in introducing a force R',which is equal and opposite to the force R, and in a direction toutilize the compressive strength of the tool to an even greater degreethan the tool 71. The tool 76 also provides greater mass at the cuttingarea than the tool 74 to conduct heat away from the cutting edge. As theangle B becomes more negative, as shown by the dashed lines and theangle -B', still more energy is wasted because of this deformation asshown by the direction of the force S. Hence, while in some applicationsa small negative angle B is desired to utilize the compressive strengthof the tool and reduce the likelihood of the point chipping, and to gainadditional mass to conduct heat, its magnitude must be limited to avoidexcessive deformation of the workpiece.

Even where a positive or negative angle B is deemed desirable, a smallmagnitude is generally sufficient. For example, angles B of less thanten degrees in magnitude are most common.

FIGS. 13 through 16 show another embodiment of this invention with acutting tool 80.

The cutting tool 80 is shown without a secondary rake angle B and a sidecutting edge angle g for clarity. The cutting tool 80 has a shank 81,and a nose 82, the nose 82 having a cutting point 83, face 84, a base85, a cutting edge 86, and an upper front edge 87. Because the secondaryrake angle B is zero, the primary rake angle A is the same as theeffective rake angle C shown in FIG. 13. In this orientation of the toolin FIGS. 13 through 16, the workpiece being machined moves past the toolin a downward direction as viewed in FIGS. 13, 14, and 16, and the toolis fed to the right as viewed in FIG. 15 during machining.

As best shown in FIG. 14 the side relief surface generally designated as90, rather than being formed in a single plane to meet the face 84 atthe cutting edge 86 as with the tool 20, is ground away at its upper endjust beneath the cutting edge 86 to form a narrow side land 91. The sideland 91 has a width designated X₁ shown in FIG. 14 and has a side reliefangle of substantially zero. The remaining portion 92 of the side reliefsurface 90, and which lies just beneath the side land 91, has a siderelief angle h₁.

In a similar manner, and as best shown in FIGS. 13 and 16, the tool 80has an end relief surface 95 having an upper portion 96 near the point83 formed in what will be referred to as an end land having a widthdesignated X₂ and a substantially zero end relief angle. The remainingportion 98 of the end relief surface 95 is formed with an end reliefangle j₁.

It will be noted that while the side land 91 extends substantially allthe way across the top of the nose 82, the end land 96 is only a smallgenerally triangular surface, formed only near the point 83. This isbecause substantially the entire side land 91 is in contact with theworkpiece during the machining process, whereas only a very smallportion at the top of the end relief surface, the size of the end land96, actually contacts the workpiece because of the end cutting edgeangle i of the tool 80 (see FIG. 15).

The reason for forming the side and end relief surfaces 90 and 95 withland surfaces, and relief angles as shown is twofold. First, formationof the cutting tool in this manner strengthens the cutting edge 86 andpoint 83 to prevent chipping under the tremendous loads created duringthe machining process. This is particularly important where the cuttingtool is made of a very hard and brittle material such as a carbide tool,although certainly superior results are achieved with tool materialswith less hardness such as high-speed steel tools as well. A secondimportant reason involves the machining of relatively high abrasionmaterials such as, for example, AISI 4340 Steel.

When machining high abrasive materials, such as those of the typementioned, the chip from the material as it is formed during themachining process wears away the faces of the cutting tool near thecutting edge 86. This erosion continues to create a crater on the face84 of the cutting tool. The dashed lines generally shown as 100 of FIG.16 show the cutting tool near its cutting edge at various stages oferosion.

With cutting tools having negative effective cutting angles orrelatively small effective cutting angles such as, for example, belowten degrees, chips from an abrasive material create forces in adirection which tend to break off the cutting edge. When this occurs,the tool quickly fails, thus greatly reducing the life of the tool.

In addition to eroding the tool on its face 84, the abrasive workpiecealso erodes the side relief surface 90 near the cutting edge 86, and theend relief surface 95 near the point 83 since these areas are in contactwith the workpiece during the machining operation. The dashed lines 105and solid lines 106 of FIG. 13, respectively, show these latter types oferosion at various stages during the machining process. Erosion of theside and end relief surfaces 90 and 95 will occur regardless of whetherthe side and end lands and relief surfaces are formed in accordance withthis embodiment of the invention. However, if not so formed, it has beenfound that as these erosions occur, and if the relief angles of the sideand end relief surfaces are improperly selected, either the erosion 100on the face of the tool will occur at too great a rate as compared tothe erosions 105 and 106 on the side and end relief surfaces 90 and 95,tending to sharpen the cutting edge so that it will eventually beunsupported, break off and fail, or the erosions 105 and 106 will be toogreat relative to the erosion 100 causing the widths X₁ and X₂ of theside and end lands 91 and 96 to become greater and greater. When thesewidths become too great, the cutting tool begins to rub and chatter, andfinally fails.

Hence, in accordance with this invention, the secondary side and endrelief angles h₁ and j₁ are selected such that the widths of the sideand end lands 91 and 96 become constant during the machining process. Inother words, with the proper selection of the angles h₁ and j₁ and thewidths X₁ and X₂, cratering of the face 84, and erosion of the reliefsurfaces 90 and 95 balance each other to produce a constant cutting edge86 under the abrasive action of the workpiece.

The widths X₁ and X₂ for a given tool material and a given workpiecematerial are dependent on the feed and speed used in the machiningoperation. The greater the speed and feed, the greater the erosions ofthe lands 91 and 96, and the greater the widths should be. But generallyfor a given feed and speed, the widths X₁ and X₂ should be selected soas not to be so small that the cutting edge will chip under the heavyloads during machining, and yet not so large as to cause rubbing,chatter, and almost immediate tool failure. Examples of values for X₁and X₂ are given in actual test results hereinafter described.

With the widths X₁ and X₂ established, it is still necessary tocorrectly define the side and end relief angles h₁ and j₁. As statedpreviously, the side and end relief angles h₁ and j₁ are correctlyselected when the widths X₁ and X₂ become constant throughout themachining operation. Referring to the enlarged view of FIG. 16, andremembering that the erosion rate is fixed for a given combination oftool and workpiece materials and feed and speed, it can be seen that ifthe side relief angle h₁ is too small, X₁ will continue to increaseduring the machining operation until it becomes so large that rubbingand chattering occur to produce tool failure. If, on the other hand, theangle h₁ is too great, the width X₁ will decrease during the machiningoperation until the cutting edge 86 becomes weak and breaks. Hence, toselect the proper angle h₁ for a given tool material, a given workpiecematerial and for given feed and speed, it is only necessary on a trialand error basis to selectively vary the angle h₁ until the width X₁remains constant. The same procedure is used to select the proper endrelief angle j₁. Examples giving specific values for h₁ and j₁ are setforth in test results to be described.

The foregoing has been a description of a cutting tool of this inventionand a method of defining the effective rake angle and relief angles ofthe cutting tool in accordance with this invention. The following isoffered as the theory or scientific principles explaining why theinvention works as described for a better appreciation for theadvancement it represents in the art.

FIG. 17 is a composite drawing showing a lathe tool nose such as, forexample, the nose 22 of the tool 20 or the nose 82 of the tool 80. Alsoshown is a workpiece W and a chip P. The workpiece W is moving in adownward direction as viewed in FIG. 17 into the cutting edge of thetool. Superimposed thereon is a portion of a tensile test specimen suchas the specimen 45 of FIG. 5 prior to loading and a sectional view ofthe test specimen after having been loaded under tension to failure. Thecup cone break profile 52, previously described with reference to FIG.6, is arranged so as to coincide with the chip failure line 110. Thedirection of the chip failure line 110 and the coincidental cup failureline is designated by the angle F with respect to a line 111 at thepoint of the tool which line is parallel to the axis of the workpiece Wand test specimen 45. The line 111 is the same as the X axis of FIG. 4.The angle F is constant for a given material and is generally acceptedto be forty-five degrees, although variations in grain size may makethis angle appear to deviate from this value. Insofar as thisexplanation is concerned, it is only important that the angle F is aconstant for a given material.

Also shown on the test specimen 45 prior to loading, are the points 56and 57, the distance between these points being represented as theinitial length l₁, and the points 56' and 57' located in the neck-downarea of the test specimen after loading in tension to failure, thedistance between these points being the true length l_(t). These are thesame points shown and described with reference to FIGS. 5 through 8.

In accordance with this invention, the cosine of the effective rakeangle C is said to be equal to the ratio of l₁ /l_(t). Referring to theenlarged view of FIG. 18, the length l_(i) in the cup cone area of thetensile specimen 45 prior to loading will translate from points 56 and57 to points 56' and 57' or to a true length of l_(t) after the tensilespecimen is loaded to failure. This new length L_(t) is the measure ofthe true elongation, and failure of the test specimen will not occuruntil the length l_(i) has been elongated to a length l_(t). When thelength l_(t) is reached under the influence of both linear elongationand neck down under tensile loading, failure occurs. A halt in tensileforces just prior to attaining a length l_(t) results in an unbrokentensile specimen. Correspondingly, the instant the tensile force isgreat enough that the length l_(t) is attained, failure is irreversibleand complete. The amount of work necessary to achieve the length ofl_(t) is the measure of the minimum work expended to break the tensilespecimen.

Like in the test specimen 45, for failure to occur in the workpiece Wsuch as required in machining, the material being machined away must bemoved or elongated from an initial length l_(i) to a true length l_(t).Otherwise, failure will not occur and the workpiece will not bemachined. If the initial width of the chip from the workpiece beforefailure, which is also the same as the feed of the tool, is l_(i) and istranslated to a new length l_(t), the minimum work has been accomplishedto create a condition of failure along the line 110. Because the initiallength l_(i) must be translated to a true length l_(t) as shown in FIG.18, the effective rake angle C of the cutting tool is automaticallydefined as that angle between l_(i) and l_(t), or cos C = l_(i) /l_(t).Hence, a cutting tool having an effective rake angle C such that cos C =l_(i) /l_(t) is the exact effective rake angle required to elongate theinitial length l_(i) to a length l_(t), which in turn is the minimumthat must be done to produce failure in and machining of the workpiece.A cutting tool having an effective rake angle C so defined will requireminimum work to produce the failure, resulting in greater efficiency andtool life.

As a corollary to this it has been found that the metal of the tensilespecimen 45 in the cup cone area 52, after being tensile loaded tofailure is much harder than the metal elsewhere on the specimen. Infact, it has been found that the metal in the cup cone area has beenworked hardened to a maximum hardness for the particular metal. It hasalso been found, that where the proper angle C is used for the cuttingtool, the chip P formed therewith is work hardened to the maximumhardness for the particular material of the workpiece. This is apparentfrom the analysis of FIGS. 17 and 18 and when it is remembered that thefailure characteristics of workpiece during machining are the failurecharacteristicis of the specimen in the area of the cup cone whentensile loaded to failure. It is believed that excessive work inproducing failure generates additional heat and deforms the granularstructure of metal to reduce the hardness of the chip. Hence, accordingto this invention, the proper angle C is the minimum angle that producesmaximum hardness of chips. This can be determined by incrementallyincreasing the angle C during machining of a particular metal andmeasuring the hardness of the chips. When the chips become no harder,that value of C is the proper one.

Although the foregoing is believed to be an accurate description of theunderlying theory of why the invention works in the manner described toproduce a vastly superior cutting tool with greatly increased tool life,its superiority has been demonstrated in actual tests. In these testscarbide tools having geometries defined in accordance with thisinvention were compared with carbide tools with the conventionalgeometries of the prior art.

Generally, the tests involved the machining of four different materials,namely, Type 303 stainless steel, AISI 4340 Steel heat treated toRockwell C 30- 31, AISI 1042 Steel, and titanium alloy Ti-6Al-4V. In allof the tests the cutting tools, both the conventional cutting tools andthe tools of this invention, were of tungsten carbide and specificallyV. R. Wesson Company, RAMET 1. All of the tools were ordered from aspecial lot and individual tools selected at random and ground to thespecified geometry. All tests were run on the same lathe with 0.004 inchfeed and 0.075 inch depth-of-cut. The speeds were varied depending onthe material being machined and were the speeds recommended in"Machining Data Handbook", 1972 edition, Metcut Research Association,Inc., compiled under contract with the U.S. Government, and all likediameters of like materials were turned at the same speed. No coolantswere used in any of the tests.

The geometry used on the conventional tools were as specified in theabove-referenced handbook, and were the same for all conventional toolsregardless of which of the four materials were to be machined. Thegeometry of each and every conventional tool used in these tests was asfollows:

Primary rake (side rake) angle A = -5°

Secondary rake (back rake) angle B = -5°

End relief angle j = 5°

Side relief angle h = 5°

Side cutting edge angle g = 30°

End cutting edge angle i = 30°

The cutting edge angles were specified in the referenced handbook as 15°but were changed to accommodate the tool holder used. This change isbelieved not to have affected the test results and in any case the samecutting edge angles were used on the tools of this invention.

For each test, that is, for each one of the four materials, all of theworkpieces were of equal size and cut from a single bar stock.Additionally, the test samples loaded under tension to failure and usedin determining the tool geometries for the tools of this invention werecut from the same single bar stock.

For each of the four tests, samples from the bar stock were made similarto the test bars 45 and 62 and loaded under tension to failure asheretofore described. The neck down angle D, the initial length l_(i)and the final length l_(f) were measured. From these measurements theeffective rake angle C was calculated in accordance with this inventionand the tools of this invention ground accordingly. The side and endrelief angles h₁ and j₁, and the widths X₁ and X₂ of the side and endlands 91 and 96 were chosen from previous tests in the manner heretoforedescribed. In accordance with this invention the relief angles and landwidths were not the same for each of the tests because of the differentworkpiece material being machined and the different speeds used. Therelief angles for the side and end lands 91 and 96 were substantiallyzero for all tests. All results are given in length of bar cut ininches. Unless otherwise stated, each tool was driven to destruction.The term "still cutting" next to a number means the tool was stillcutting when the test was halted as when the bar was used up.

TEST 1

The material machined was Type 303 stainless steel. The geometry of theconventional tool was as heretofore described. The geometry of thisinvention was as follows:

Effective rake angle C = 45°

Primary rake angle A = 46°

Secondary rake angle B = 0°

Side and end land widths X₁ and X₂ = 0.005 in.

Side and end relief angles h₁ and j₁ = 15°

Four of each type tool were tested. The results are tabulated asfollows:Conventional Tool Tool Of ThisInvention______________________________________ 1. 661/4 5. 1505/8(still cutting) 2. 513/8 6. 1461/8 (still cutting) 3. 377/8 7. 139(still cutting) 4. 1477/8 (still cutting) 8. 5Totals 3033/84403/4______________________________________

The reason for the poor results for Tool No. 8 is believed to be thatthe tool was defective.

TEST 2

The material machined was AISI 4340 Steel heat treated to Rockwell C 30-31. The geometry of the conventional tool was as heretofore described.The geometry of the tool of this invention was as follows:

Effective rake angle C = 40°

Primary rake angle A = 40°

Secondary rake angle B = 0°

Side and end land widths X₁ and X₂ = 0.010 in.

Side and end relief angles h₁ and j₁ = 22°

Five conventional tools and four tools of this invention were tested.The results are tabulated as follows:Conventional Tool Tool Of ThisInvention______________________________________1. 221/2 6. 307/8 (stillcutting)2. 183/4 7. 343/8 (still cutting)3. 12 8. 291/4 (stillcutting)4. 25 9. 341/2 (still cutting)5. 211/2Totals 5) 993/4 4) 129 20321/4 (still cutting)______________________________________

It should be noted that all of the conventional tools failed whereas allof the tools of this invention were still cutting when the test washalted. The average tool life of the conventional tool was twenty inchesand for the tool of this invention thirty-two and one-fourth inches andstill cutting.

TEST 3

The material machined was AISI 1042 Steel. The geometry of theconventional tool was as heretofore described. The geometry of the toolof this invention was as follows:

Effective rake angle C = 44°

Primary rake length A = 44°

Secondary rake angle B = 0°

Side and end land widths X₁ and X₂ = 0.005 in.

Side and end relief angles h₁ and j₁ = 20°

Four tools of each type were tested and the results are tabulated asfollows:

    Conventional Tool                                                                              Tool Of This Invention                                       ______________________________________                                                 1.    141/4     5.  39   (still cutting)                                      2.    135/8     6.  39   (still cutting)                                      3.     81/2     7.  39   (still cutting)                                      4.    133/8     8.  39   (still cutting)-Totals  493/4  156 (stil                                      l cutting)                                  ______________________________________                                    

Again it should be noted that while all of the conventional toolsfailed, all of the tools of this invention were still cutting when thetest was halted.

TEST 4

The material machined was titanium alloy Ti-6Al-4V. This material waschosen because it is very difficult to machine. The geometry of theconventional tool was as heretofore described. The geometries of thetools of this invention were as follows:

For Tools 4 and 5

Effective rake angle C = 33°

Primary rake angle A = 33°

Secondary rake angle B = 0°

Side and end land widths X₁ and X₂ = 0.005 in.

Side and end relief angles h₁ and j₁ = 20°

For Tools 6 and 7

Effective rake angle C = 33°

Primary rake angle A = 32°

Secondary rake angle B = -121/2°

Side and end land widths X₁ and X₂ = 0.005 in.

Side and end relief angles h₁ and j₁ = 20°

This test was verified by an independent test laboratory. Threeconventional tools and four tools of this invention were tested and theresults are tabulated as follows:

           Conventional Tool                                                                         Tool Of This Invention                                     ______________________________________                                               1.     763/8      4.      4243/8                                              2.    1001/8      5.      1727/8                                              3.     903/4      6.      1441/2                                                                7.      1361/2                                       Totals     3)    2671/4        4)  8781/4                                                       89               219.5                                      ______________________________________                                    

There are several things to note from these test results. One is theradical departures from the rake angles and relief angles taught in theprior art for use on tungsten carbide tools. While the referencedhandbook specifies an effective rake angle of -7° for all thesematerials, the effective rake angles for the tools of this invention formachining these materials range from 32° to 46°a difference of from 39°to 53°. The relief angles taught by the referenced handbook formachining these materials are 5°, while the relief angles of the toolsof this invention for these same materials range from 15° to 22°. Theuse of such high rake and relief angles on carbide tools, and generallytools of high red hardness, goes directly against the teachings of theprior art.

This is further exemplified by the graph of FIG. 19 showing plots ofeffective rake angle C in degrees versus the true elongation in percent,or ##EQU1## for metals to be machined. The curve 120 shows thisrelationship for the tools of this invention, including carbide tools,where cos C = l_(i) /l_(t).

The curve 120 starts at zero indicating an angle C of zero for materialshaving no true elongation, or zero ductility, and proceeds in a positivedirection. The slope of the curve 120 is relatively small below 10percent true elongation and becomes increasingly larger above 10percent. By far most of the metals, except for the very brittle such asthe more brittle cast irons and high carbon steels, have trueelongations about 10 percent. These would include mild and alloyedsteels and most of the nonferrous metals such as aluminum, stainlesssteel, copper, titanium, magnesium, etc. The angle C according to thecurve 120 is always positive and at least 24° for all metals of over 10percent true elongation.

In contrast, the points 121 through 124 show the effective rake angles Cfor the carbide tools recommended by the "Machining Data Handbook"previously referenced for cutting the metals used in the testsheretofore described. These angles are each a negative seven degreesthus exemplifying the radical departure this invention represents overthe prior art.

Hence, there has been described a novel cutting tool having a geometrythat machines a metal workpiece with minimum work and maximum efficiencyfor greatly improved tool life, and a novel method of determining thecorrect geometry for the tool.

Various changes and modifications may be made in this invention, as willbe readily apparent to those skilled in the art. Such changes andmodifications are within the scope and teaching of this invention asdefined by the claims appended hereto.

What is claimed is:
 1. A method of making a cutting tool for machining aselected metal comprising the steps of forming the tool with a cuttingedge made from a material that retains its cutting effectiveness attemperatures over about 1200° F., and with a side wear land surface anda side relief surface, the relief angle of the side land surface beingsubstantially zero and the relief angle of the side relief surface beingof a selected value, using the tool to machine the metal, andselectively changing the width of the side land surface and themagnitude of the side relief angle as needed such that the width of theside land surface becomes substantially constant as the tool is used tomachine the metal.
 2. The method of claim 1 further comprising the stepof forming the tool with an end wear land surface and an end reliefsurface, the relief angle of the end wear land surface beingsubstantially zero and the relief angle of the end relief surface beingof a selected value, and selectively changing the width of the end landsurface and the magnitude of the end relief angle as needed such thatthe width of the end wear land surface becomes substantially constant asthe tool is used in machining the metal.
 3. The method of claim 1further comprising the step of forming the width of the side landsurface so as to be not so narrow that the tool cutting edge chipsduring machining or so wide as to cause rubbing or chattering duringmachining.
 4. The method of claim 1 wherein the forming step furthercomprises forming the tool with the relief angle of the side landsurface being approximately no greater than 22°, and wherein thechanging step further comprises the step of increasing the magnitude ofthe side relief angle as needed such that the width of the side landsurface becomes substantially constant as the tool is used to machinethe metal.
 5. The method of claim 1 further comprising the step offorming the tool with an effective rake angle defined substantially inaccordance with the formula cos C = (l_(i) /l_(f)) cos D, where C is theeffective rake angle of the cutting tool, l_(i) /l_(f) is the ratio ofthe initial length to final length of the specimen of the metal to bemachined when tensile loaded to failure measured parallel to the appliedload, and D is the neck down angle of the specimen in the failure areaafter being tensile loaded to failure.